Endoplasmic Reticulum: Structure and Function

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Endoplasmic Reticulum

Endoplasmic Reticulum
The endoplasmic reticulum plays an important role in the biosynthesis, processing, and transport of proteins and lipids. Credit: Encyclopaedia Britannica/UIG/Getty Images

Endoplasmic Reticulum

The endoplasmic reticulum (ER) is an important organelle in eukaryotic cells. It plays a major role in the production, processing, and transport of proteins and lipids. The ER produces transmembrane proteins and lipids for its membrane and for many other cell components including lysosomes, secretory vesicles, the Golgi appatatus, the cell membrane, and plant cell vacuoles.

The endoplasmic reticulum is a network of tubules and flattened sacs that serve a variety of functions in plant and animal cells. There are two regions of the ER that differ in both structure and function. One region is called rough ER because it has ribosomes attached to the cytoplasmic side of the membrane. The other region is called smooth ER because it lacks attached ribosomes. Typically, the smooth ER is a tubule network and the rough ER is a series of flattened sacs. The space inside of the ER is called the lumen. The ER is very extensive extending from the cell membrane through the cytoplasm and forming a continuous connection with the nuclear envelope. Since the ER is connected with the nuclear envelope, the lumen of the ER and the space inside the nuclear envelope are part of the same compartment.

Rough Endoplasmic Reticulum

The rough endoplasmic reticulum manufactures membranes and secretory proteins. The ribosomes attached to the rough ER synthesize proteins by the process of translation. In certain leukocytes (white blood cells), the rough ER produces antibodies. In pancreatic cells, the rough ER produces insulin. The rough and smooth ER are usually interconnected and the proteins and membranes made by the rough ER move into the smooth ER to be transferred to other locations. Some proteins are sent to the Golgi apparatus by special transport vesicles. After the proteins have been modified in the Golgi, they are transported to their proper destinations within the cell or exported from the cell by exocytosis.

Smooth Endoplasmic Reticulum

The smooth ER has a wide range of functions including carbohydrate and lipid synthesis. Lipids such as phospholipids and cholesterol are necessary for the construction of cell membranes. Smooth ER also serves as a transitional area for vesicles that transport ER products to various destinations. In liver cells the smooth ER produces enzymes that help to detoxify certain compounds. In muscles the smooth ER assists in the contraction of muscle cells, and in brain cells it synthesizes male and female hormones.

Eukaryotic Cell Structures

The endoplasmic reticulum is only one component of a cell. The following cell structures can also be found in a typical animal eukaryotic cell: